Split View: 이벤트 드리븐 아키텍처 패턴 가이드 2025: CQRS, Event Sourcing, Saga 패턴까지

이벤트 유형
├── Domain Event (도메인 이벤트)
│   └── 비즈니스 도메인에서 발생한 사실
│       예: OrderPlaced, PaymentFailed
├── Integration Event (통합 이벤트)
│   └── 바운디드 컨텍스트 간 통신을 위한 이벤트
│       예: OrderConfirmedIntegrationEvent
├── Event Notification (이벤트 알림)
│   └── 최소한의 데이터만 포함. 소비자가 필요시 조회
│       예: OrderStatusChanged (orderId만 포함)
└── Event-Carried State Transfer (상태 전달 이벤트)
    └── 전체 상태를 포함. 소비자가 별도 조회 불필요
        예: OrderDetails (전체 주문 정보 포함)

// TypeScript - 이벤트 버스 인터페이스
interface EventBus {
  publish<T extends DomainEvent>(event: T): Promise<void>;
  subscribe<T extends DomainEvent>(
    eventType: string,
    handler: EventHandler<T>
  ): void;
  unsubscribe(eventType: string, handler: EventHandler<unknown>): void;
}

interface DomainEvent {
  eventId: string;
  eventType: string;
  aggregateId: string;
  occurredAt: Date;
  version: number;
  metadata: Record<string, string>;
}

interface EventHandler<T extends DomainEvent> {
  handle(event: T): Promise<void>;
}

// 기본 도메인 이벤트
abstract class BaseDomainEvent implements DomainEvent {
  readonly eventId: string;
  readonly occurredAt: Date;
  readonly version: number;
  abstract readonly eventType: string;
  abstract readonly aggregateId: string;
  metadata: Record<string, string> = {};

  constructor() {
    this.eventId = crypto.randomUUID();
    this.occurredAt = new Date();
    this.version = 1;
  }
}

// 주문 생성 이벤트
class OrderPlaced extends BaseDomainEvent {
  readonly eventType = 'order.placed';
  readonly aggregateId: string;

  constructor(
    public readonly orderId: string,
    public readonly customerId: string,
    public readonly items: OrderItem[],
    public readonly totalAmount: Money,
    public readonly shippingAddress: Address
  ) {
    super();
    this.aggregateId = orderId;
  }
}

// 결제 완료 이벤트
class PaymentCompleted extends BaseDomainEvent {
  readonly eventType = 'payment.completed';
  readonly aggregateId: string;

  constructor(
    public readonly paymentId: string,
    public readonly orderId: string,
    public readonly amount: Money,
    public readonly paymentMethod: string
  ) {
    super();
    this.aggregateId = paymentId;
  }
}

// 재고 예약 이벤트
class InventoryReserved extends BaseDomainEvent {
  readonly eventType = 'inventory.reserved';
  readonly aggregateId: string;

  constructor(
    public readonly reservationId: string,
    public readonly orderId: string,
    public readonly items: ReservedItem[]
  ) {
    super();
    this.aggregateId = reservationId;
  }
}

// 이벤트 스키마 (Avro-like)
const OrderPlacedSchema = {
  type: 'record',
  name: 'OrderPlaced',
  namespace: 'com.ecommerce.orders.v1',
  fields: [
    { name: 'eventId', type: 'string' },
    { name: 'eventType', type: 'string' },
    { name: 'aggregateId', type: 'string' },
    { name: 'occurredAt', type: 'string' },  // ISO 8601
    { name: 'version', type: 'int' },
    { name: 'orderId', type: 'string' },
    { name: 'customerId', type: 'string' },
    {
      name: 'items',
      type: {
        type: 'array',
        items: {
          type: 'record',
          name: 'OrderItem',
          fields: [
            { name: 'productId', type: 'string' },
            { name: 'quantity', type: 'int' },
            { name: 'unitPrice', type: 'long' },
          ],
        },
      },
    },
    { name: 'totalAmount', type: 'long' },
    { name: 'currency', type: 'string' },
  ],
};

전통적 방식 (CRUD):
┌─────────────────────────┐
│ orders 테이블            │
│ id: ORD-001             │
│ status: shipped         │ ← 현재 상태만 저장
│ total: 50000            │
│ updated_at: 2025-03-24  │
└─────────────────────────┘

Event Sourcing 방식:
┌─────────────────────────────────────────┐
│ event_store                             │
│ 1. OrderPlaced    (ORD-001, 2025-03-24) │
│ 2. PaymentReceived(ORD-001, 2025-03-24) │
│ 3. ItemPacked     (ORD-001, 2025-03-24) │
│ 4. OrderShipped   (ORD-001, 2025-03-24) │ ← 모든 이력 보존
└─────────────────────────────────────────┘

// 이벤트 스토어 인터페이스
interface EventStore {
  // 이벤트 추가 (낙관적 동시성 제어)
  append(
    streamId: string,
    events: DomainEvent[],
    expectedVersion: number
  ): Promise<void>;

  // 스트림에서 이벤트 읽기
  readStream(
    streamId: string,
    fromVersion?: number
  ): Promise<DomainEvent[]>;

  // 전체 이벤트 구독
  subscribe(
    handler: (event: DomainEvent) => Promise<void>
  ): void;
}

// PostgreSQL 기반 이벤트 스토어
class PostgresEventStore implements EventStore {
  async append(
    streamId: string,
    events: DomainEvent[],
    expectedVersion: number
  ): Promise<void> {
    const client = await this.pool.connect();
    try {
      await client.query('BEGIN');

      // 낙관적 동시성 검사
      const result = await client.query(
        'SELECT MAX(version) as current_version FROM events WHERE stream_id = $1',
        [streamId]
      );
      const currentVersion = result.rows[0]?.current_version ?? 0;

      if (currentVersion !== expectedVersion) {
        throw new ConcurrencyError(
          `Expected version ${expectedVersion}, but current is ${currentVersion}`
        );
      }

      // 이벤트 저장
      let version = expectedVersion;
      for (const event of events) {
        version++;
        await client.query(
          `INSERT INTO events (event_id, stream_id, event_type, data, metadata, version, created_at)
           VALUES ($1, $2, $3, $4, $5, $6, $7)`,
          [
            event.eventId,
            streamId,
            event.eventType,
            JSON.stringify(event),
            JSON.stringify(event.metadata),
            version,
            event.occurredAt,
          ]
        );
      }

      await client.query('COMMIT');

      // 이벤트 발행 (비동기)
      for (const event of events) {
        await this.eventBus.publish(event);
      }
    } catch (error) {
      await client.query('ROLLBACK');
      throw error;
    } finally {
      client.release();
    }
  }

  async readStream(streamId: string, fromVersion = 0): Promise<DomainEvent[]> {
    const result = await this.pool.query(
      'SELECT data FROM events WHERE stream_id = $1 AND version > $2 ORDER BY version ASC',
      [streamId, fromVersion]
    );
    return result.rows.map((row) => JSON.parse(row.data));
  }
}

// Event Sourced Aggregate
abstract class EventSourcedAggregate {
  private uncommittedEvents: DomainEvent[] = [];
  private _version: number = 0;

  get version(): number {
    return this._version;
  }

  // 이벤트 적용 (상태 변경)
  protected apply(event: DomainEvent): void {
    this.when(event);
    this.uncommittedEvents.push(event);
    this._version++;
  }

  // 이벤트 핸들러 (하위 클래스에서 구현)
  protected abstract when(event: DomainEvent): void;

  // 이벤트로부터 상태 복원
  loadFromHistory(events: DomainEvent[]): void {
    for (const event of events) {
      this.when(event);
      this._version++;
    }
  }

  getUncommittedEvents(): DomainEvent[] {
    return [...this.uncommittedEvents];
  }

  clearUncommittedEvents(): void {
    this.uncommittedEvents = [];
  }
}

// 주문 Aggregate
class Order extends EventSourcedAggregate {
  private id: string = '';
  private customerId: string = '';
  private items: OrderItem[] = [];
  private status: OrderStatus = OrderStatus.DRAFT;
  private totalAmount: number = 0;

  // 명령: 주문 생성
  static create(
    orderId: string,
    customerId: string,
    items: OrderItem[]
  ): Order {
    const order = new Order();
    const totalAmount = items.reduce(
      (sum, item) => sum + item.unitPrice * item.quantity, 0
    );

    order.apply(new OrderPlaced(orderId, customerId, items, totalAmount));
    return order;
  }

  // 명령: 결제 확인
  confirmPayment(paymentId: string, amount: number): void {
    if (this.status !== OrderStatus.PLACED) {
      throw new Error('Order is not in placed status');
    }
    this.apply(new OrderPaymentConfirmed(this.id, paymentId, amount));
  }

  // 명령: 주문 취소
  cancel(reason: string): void {
    if (this.status === OrderStatus.SHIPPED) {
      throw new Error('Cannot cancel shipped order');
    }
    this.apply(new OrderCancelled(this.id, reason));
  }

  // 이벤트 핸들러
  protected when(event: DomainEvent): void {
    if (event instanceof OrderPlaced) {
      this.id = event.orderId;
      this.customerId = event.customerId;
      this.items = event.items;
      this.totalAmount = event.totalAmount;
      this.status = OrderStatus.PLACED;
    } else if (event instanceof OrderPaymentConfirmed) {
      this.status = OrderStatus.CONFIRMED;
    } else if (event instanceof OrderCancelled) {
      this.status = OrderStatus.CANCELLED;
    }
  }
}

class SnapshotStore {
  async saveSnapshot(
    streamId: string,
    snapshot: AggregateSnapshot
  ): Promise<void> {
    await this.pool.query(
      `INSERT INTO snapshots (stream_id, version, data, created_at)
       VALUES ($1, $2, $3, NOW())
       ON CONFLICT (stream_id) DO UPDATE SET version = $2, data = $3, created_at = NOW()`,
      [streamId, snapshot.version, JSON.stringify(snapshot.state)]
    );
  }

  async getLatestSnapshot(streamId: string): Promise<AggregateSnapshot | null> {
    const result = await this.pool.query(
      'SELECT version, data FROM snapshots WHERE stream_id = $1',
      [streamId]
    );
    if (result.rows.length === 0) return null;
    return {
      version: result.rows[0].version,
      state: JSON.parse(result.rows[0].data),
    };
  }
}

// 스냅샷을 활용한 Aggregate 로딩
class OrderRepository {
  async getById(orderId: string): Promise<Order> {
    const order = new Order();

    // 1. 스냅샷 로드
    const snapshot = await this.snapshotStore.getLatestSnapshot(orderId);
    let fromVersion = 0;

    if (snapshot) {
      order.restoreFromSnapshot(snapshot.state);
      fromVersion = snapshot.version;
    }

    // 2. 스냅샷 이후의 이벤트만 로드
    const events = await this.eventStore.readStream(orderId, fromVersion);
    order.loadFromHistory(events);

    // 3. 일정 이벤트 수 이후 스냅샷 생성
    if (events.length > 50) {
      await this.snapshotStore.saveSnapshot(orderId, {
        version: order.version,
        state: order.toSnapshot(),
      });
    }

    return order;
  }
}

// 주문 조회 프로젝션
class OrderProjection {
  async handle(event: DomainEvent): Promise<void> {
    if (event instanceof OrderPlaced) {
      await this.db.query(
        `INSERT INTO order_read_model (id, customer_id, status, total_amount, created_at)
         VALUES ($1, $2, $3, $4, $5)`,
        [event.orderId, event.customerId, 'placed', event.totalAmount, event.occurredAt]
      );

      // 주문 아이템 프로젝션
      for (const item of event.items) {
        await this.db.query(
          `INSERT INTO order_items_read_model (order_id, product_id, quantity, unit_price)
           VALUES ($1, $2, $3, $4)`,
          [event.orderId, item.productId, item.quantity, item.unitPrice]
        );
      }
    } else if (event instanceof OrderPaymentConfirmed) {
      await this.db.query(
        'UPDATE order_read_model SET status = $1, payment_id = $2, updated_at = $3 WHERE id = $4',
        ['confirmed', event.paymentId, event.occurredAt, event.orderId]
      );
    } else if (event instanceof OrderShipped) {
      await this.db.query(
        'UPDATE order_read_model SET status = $1, tracking_number = $2, shipped_at = $3 WHERE id = $4',
        ['shipped', event.trackingNumber, event.occurredAt, event.orderId]
      );
    }
  }
}

┌──────────────┐     Command      ┌──────────────┐
│   Client     │ ───────────────► │ Command Side │
│              │                  │  (Write)     │
│              │     Query        │              │
│              │ ───────────────► │              │
└──────────────┘                  └──────┬───────┘
                                         │
                                    Events│
                                         ▼
                                  ┌──────────────┐
                                  │  Event Bus   │
                                  └──────┬───────┘
                                         │
                                         ▼
┌──────────────┐     Query        ┌──────────────┐
│   Client     │ ◄─────────────── │  Query Side  │
│              │                  │  (Read)      │
└──────────────┘                  └──────────────┘

// Command 정의
interface Command {
  commandId: string;
  timestamp: Date;
}

class PlaceOrderCommand implements Command {
  commandId: string;
  timestamp: Date;

  constructor(
    public readonly customerId: string,
    public readonly items: OrderItemDto[],
    public readonly shippingAddress: AddressDto
  ) {
    this.commandId = crypto.randomUUID();
    this.timestamp = new Date();
  }
}

// Command Handler
class PlaceOrderHandler {
  constructor(
    private orderRepository: OrderRepository,
    private inventoryService: InventoryService,
    private pricingService: PricingService
  ) {}

  async handle(command: PlaceOrderCommand): Promise<string> {
    // 1. 재고 확인
    await this.inventoryService.checkAvailability(command.items);

    // 2. 가격 계산
    const pricedItems = await this.pricingService.calculatePrices(command.items);

    // 3. 주문 Aggregate 생성
    const orderId = crypto.randomUUID();
    const order = Order.create(orderId, command.customerId, pricedItems);

    // 4. 이벤트 스토어에 저장
    await this.orderRepository.save(order);

    return orderId;
  }
}

// Command Bus
class CommandBus {
  private handlers: Map<string, CommandHandler<unknown>> = new Map();

  register<T extends Command>(
    commandType: string,
    handler: CommandHandler<T>
  ): void {
    this.handlers.set(commandType, handler);
  }

  async dispatch<T extends Command>(command: T): Promise<unknown> {
    const handler = this.handlers.get(command.constructor.name);
    if (!handler) {
      throw new Error(`No handler for ${command.constructor.name}`);
    }
    return handler.handle(command);
  }
}

// Query 정의
class GetOrderDetailsQuery {
  constructor(public readonly orderId: string) {}
}

class ListCustomerOrdersQuery {
  constructor(
    public readonly customerId: string,
    public readonly page: number = 1,
    public readonly pageSize: number = 20
  ) {}
}

// Query Handler
class GetOrderDetailsHandler {
  constructor(private readDb: Pool) {}

  async handle(query: GetOrderDetailsQuery): Promise<OrderDetailsDto> {
    const result = await this.readDb.query(
      `SELECT o.*, json_agg(i.*) as items
       FROM order_read_model o
       JOIN order_items_read_model i ON o.id = i.order_id
       WHERE o.id = $1
       GROUP BY o.id`,
      [query.orderId]
    );

    if (result.rows.length === 0) {
      throw new NotFoundError(`Order ${query.orderId} not found`);
    }

    return this.mapToDto(result.rows[0]);
  }
}

// 최종 일관성 (Eventual Consistency)
// 읽기 모델은 이벤트 처리 후 업데이트되므로 약간의 지연이 있을 수 있음
class OrderQueryService {
  async getOrderWithConsistencyCheck(
    orderId: string,
    expectedVersion?: number
  ): Promise<OrderDetailsDto> {
    const order = await this.getOrderDetails(orderId);

    // 특정 버전이 요구되면 확인
    if (expectedVersion && order.version < expectedVersion) {
      // 프로젝션이 아직 따라잡지 못한 경우
      // 잠시 대기 후 재시도 또는 Command 측에서 직접 조회
      await this.waitForProjection(orderId, expectedVersion);
      return this.getOrderDetails(orderId);
    }

    return order;
  }
}

Choreography (안무 방식):
서비스들이 이벤트를 통해 자율적으로 협력

OrderService ──OrderPlaced──► PaymentService
                              │
                              PaymentCompleted
                              │
                              ▼
                          InventoryService
                              │
                              InventoryReserved
                              │
                              ▼
                          ShippingService

Orchestration (오케스트레이션 방식):
중앙 오케스트레이터가 전체 흐름을 조율

              ┌──────────────────┐
              │  OrderSaga       │
              │  (Orchestrator)  │
              └──────┬───────────┘
                     │
        ┌────────────┼────────────┐
        ▼            ▼            ▼
PaymentService InventoryService ShippingService

// 주문 서비스: 이벤트 발행
class OrderService {
  async placeOrder(command: PlaceOrderCommand): Promise<string> {
    const order = Order.create(command);
    await this.orderRepository.save(order);
    // OrderPlaced 이벤트가 자동 발행됨
    return order.id;
  }
}

// 결제 서비스: OrderPlaced 이벤트 처리
class PaymentEventHandler {
  @EventHandler('order.placed')
  async onOrderPlaced(event: OrderPlaced): Promise<void> {
    try {
      const payment = await this.paymentService.processPayment(
        event.orderId,
        event.totalAmount
      );
      // 성공 이벤트 발행
      await this.eventBus.publish(new PaymentCompleted(
        payment.id, event.orderId, event.totalAmount
      ));
    } catch (error) {
      // 실패 이벤트 발행 (보상 트랜잭션 트리거)
      await this.eventBus.publish(new PaymentFailed(
        event.orderId, error.message
      ));
    }
  }
}

// 재고 서비스: PaymentCompleted 이벤트 처리
class InventoryEventHandler {
  @EventHandler('payment.completed')
  async onPaymentCompleted(event: PaymentCompleted): Promise<void> {
    try {
      const reservation = await this.inventoryService.reserveItems(
        event.orderId
      );
      await this.eventBus.publish(new InventoryReserved(
        reservation.id, event.orderId, reservation.items
      ));
    } catch (error) {
      // 보상: 결제 환불 트리거
      await this.eventBus.publish(new InventoryReservationFailed(
        event.orderId, error.message
      ));
    }
  }
}

// 주문 서비스: 보상 트랜잭션 처리
class OrderCompensationHandler {
  @EventHandler('payment.failed')
  async onPaymentFailed(event: PaymentFailed): Promise<void> {
    await this.orderService.cancelOrder(event.orderId, 'Payment failed');
  }

  @EventHandler('inventory.reservation.failed')
  async onInventoryFailed(event: InventoryReservationFailed): Promise<void> {
    // 결제 환불 요청
    await this.paymentService.refund(event.orderId);
    await this.orderService.cancelOrder(event.orderId, 'Out of stock');
  }
}

// Saga 정의
class OrderSaga {
  private state: SagaState = SagaState.STARTED;
  private orderId: string;
  private paymentId?: string;
  private reservationId?: string;

  constructor(
    private commandBus: CommandBus,
    private sagaStore: SagaStore
  ) {}

  async start(orderId: string): Promise<void> {
    this.orderId = orderId;
    this.state = SagaState.PAYMENT_PENDING;
    await this.sagaStore.save(this);

    // Step 1: 결제 요청
    await this.commandBus.dispatch(new ProcessPaymentCommand(orderId));
  }

  async handlePaymentCompleted(event: PaymentCompleted): Promise<void> {
    this.paymentId = event.paymentId;
    this.state = SagaState.INVENTORY_PENDING;
    await this.sagaStore.save(this);

    // Step 2: 재고 예약 요청
    await this.commandBus.dispatch(new ReserveInventoryCommand(
      this.orderId, event.paymentId
    ));
  }

  async handlePaymentFailed(event: PaymentFailed): Promise<void> {
    this.state = SagaState.COMPENSATING;
    await this.sagaStore.save(this);

    // 보상: 주문 취소
    await this.commandBus.dispatch(new CancelOrderCommand(
      this.orderId, 'Payment failed'
    ));

    this.state = SagaState.COMPENSATED;
    await this.sagaStore.save(this);
  }

  async handleInventoryReserved(event: InventoryReserved): Promise<void> {
    this.reservationId = event.reservationId;
    this.state = SagaState.SHIPPING_PENDING;
    await this.sagaStore.save(this);

    // Step 3: 배송 요청
    await this.commandBus.dispatch(new CreateShipmentCommand(
      this.orderId, this.reservationId
    ));
  }

  async handleInventoryFailed(event: InventoryReservationFailed): Promise<void> {
    this.state = SagaState.COMPENSATING;
    await this.sagaStore.save(this);

    // 보상: 결제 환불 후 주문 취소
    await this.commandBus.dispatch(new RefundPaymentCommand(
      this.orderId, this.paymentId!
    ));
    await this.commandBus.dispatch(new CancelOrderCommand(
      this.orderId, 'Inventory unavailable'
    ));

    this.state = SagaState.COMPENSATED;
    await this.sagaStore.save(this);
  }

  async handleShipmentCreated(event: ShipmentCreated): Promise<void> {
    this.state = SagaState.COMPLETED;
    await this.sagaStore.save(this);

    // 주문 확정
    await this.commandBus.dispatch(new ConfirmOrderCommand(this.orderId));
  }
}

문제 시나리오:
1. DB에 주문 저장 ✓
2. Kafka에 이벤트 발행 ✗  ← 실패하면 데이터 불일치!

Outbox 패턴:
1. DB 트랜잭션 내에서:
   - 주문 저장 ✓
   - outbox 테이블에 이벤트 저장 ✓
2. 별도 프로세스가 outbox에서 읽어 Kafka에 발행

-- Outbox 테이블
CREATE TABLE outbox (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  aggregate_type VARCHAR(255) NOT NULL,
  aggregate_id VARCHAR(255) NOT NULL,
  event_type VARCHAR(255) NOT NULL,
  payload JSONB NOT NULL,
  created_at TIMESTAMP NOT NULL DEFAULT NOW(),
  published_at TIMESTAMP NULL,
  retry_count INT DEFAULT 0,
  last_error TEXT NULL
);

CREATE INDEX idx_outbox_unpublished ON outbox (created_at)
  WHERE published_at IS NULL;

// Outbox 패턴 구현
class OrderService {
  async placeOrder(command: PlaceOrderCommand): Promise<string> {
    const client = await this.pool.connect();

    try {
      await client.query('BEGIN');

      // 1. 주문 저장
      const orderId = crypto.randomUUID();
      await client.query(
        'INSERT INTO orders (id, customer_id, status, total_amount) VALUES ($1, $2, $3, $4)',
        [orderId, command.customerId, 'placed', command.totalAmount]
      );

      // 2. Outbox에 이벤트 저장 (같은 트랜잭션)
      const event = new OrderPlaced(orderId, command.customerId, command.items, command.totalAmount);
      await client.query(
        `INSERT INTO outbox (aggregate_type, aggregate_id, event_type, payload)
         VALUES ($1, $2, $3, $4)`,
        ['Order', orderId, 'order.placed', JSON.stringify(event)]
      );

      await client.query('COMMIT');
      return orderId;
    } catch (error) {
      await client.query('ROLLBACK');
      throw error;
    } finally {
      client.release();
    }
  }
}

# Debezium Connector 설정
apiVersion: kafka.strimzi.io/v1beta2
kind: KafkaConnector
metadata:
  name: outbox-connector
spec:
  class: io.debezium.connector.postgresql.PostgresConnector
  tasksMax: 1
  config:
    database.hostname: postgres
    database.port: 5432
    database.user: debezium
    database.password: secret
    database.dbname: orders
    table.include.list: public.outbox
    transforms: outbox
    transforms.outbox.type: io.debezium.transforms.outbox.EventRouter
    transforms.outbox.table.field.event.id: id
    transforms.outbox.table.field.event.key: aggregate_id
    transforms.outbox.table.field.event.type: event_type
    transforms.outbox.table.field.event.payload: payload
    transforms.outbox.route.by.field: aggregate_type
    transforms.outbox.route.topic.replacement: events.orders

class OutboxPollingPublisher {
  private isRunning = false;

  async start(): Promise<void> {
    this.isRunning = true;

    while (this.isRunning) {
      try {
        await this.publishPendingEvents();
      } catch (error) {
        console.error('Polling error:', error);
      }
      await this.sleep(1000); // 1초 간격
    }
  }

  private async publishPendingEvents(): Promise<void> {
    const client = await this.pool.connect();
    try {
      // 미발행 이벤트 조회 (잠금)
      const result = await client.query(
        `SELECT * FROM outbox
         WHERE published_at IS NULL AND retry_count < 5
         ORDER BY created_at ASC
         LIMIT 100
         FOR UPDATE SKIP LOCKED`
      );

      for (const row of result.rows) {
        try {
          // Kafka에 발행
          await this.kafka.produce(
            `events.${row.aggregate_type.toLowerCase()}`,
            row.aggregate_id,
            row.payload
          );

          // 발행 완료 표시
          await client.query(
            'UPDATE outbox SET published_at = NOW() WHERE id = $1',
            [row.id]
          );
        } catch (error) {
          // 실패 카운트 증가
          await client.query(
            'UPDATE outbox SET retry_count = retry_count + 1, last_error = $1 WHERE id = $2',
            [error.message, row.id]
          );
        }
      }
    } finally {
      client.release();
    }
  }
}

class IdempotencyStore {
  async isProcessed(idempotencyKey: string): Promise<boolean> {
    const result = await this.pool.query(
      'SELECT 1 FROM processed_events WHERE idempotency_key = $1',
      [idempotencyKey]
    );
    return result.rows.length > 0;
  }

  async markProcessed(
    idempotencyKey: string,
    result: unknown,
    client: PoolClient
  ): Promise<void> {
    await client.query(
      `INSERT INTO processed_events (idempotency_key, result, processed_at)
       VALUES ($1, $2, NOW())
       ON CONFLICT (idempotency_key) DO NOTHING`,
      [idempotencyKey, JSON.stringify(result)]
    );
  }
}

// 멱등성 보장 이벤트 핸들러
class IdempotentEventHandler<T extends DomainEvent> {
  constructor(
    private idempotencyStore: IdempotencyStore,
    private innerHandler: EventHandler<T>
  ) {}

  async handle(event: T): Promise<void> {
    const key = `${event.eventType}:${event.eventId}`;

    // 이미 처리된 이벤트인지 확인
    if (await this.idempotencyStore.isProcessed(key)) {
      console.log(`Event ${key} already processed, skipping`);
      return;
    }

    const client = await this.pool.connect();
    try {
      await client.query('BEGIN');

      // 이벤트 처리
      await this.innerHandler.handle(event);

      // 처리 완료 표시 (같은 트랜잭션)
      await this.idempotencyStore.markProcessed(key, null, client);

      await client.query('COMMIT');
    } catch (error) {
      await client.query('ROLLBACK');
      throw error;
    } finally {
      client.release();
    }
  }
}

// Kafka Consumer 중복 제거
class DeduplicatingConsumer {
  private recentEventIds: Set<string> = new Set();
  private maxCacheSize = 10000;

  async processMessage(message: KafkaMessage): Promise<void> {
    const event = JSON.parse(message.value.toString());
    const eventId = event.eventId;

    // 메모리 캐시에서 중복 체크 (빠른 경로)
    if (this.recentEventIds.has(eventId)) {
      return; // 중복 - 스킵
    }

    // DB에서 중복 체크 (느린 경로)
    if (await this.idempotencyStore.isProcessed(eventId)) {
      this.recentEventIds.add(eventId);
      return; // 중복 - 스킵
    }

    // 이벤트 처리
    await this.handler.handle(event);

    // 캐시 갱신
    this.recentEventIds.add(eventId);
    if (this.recentEventIds.size > this.maxCacheSize) {
      const firstKey = this.recentEventIds.values().next().value;
      this.recentEventIds.delete(firstKey);
    }
  }
}

class EventConsumerWithDLQ {
  private maxRetries = 3;

  async processEvent(event: DomainEvent, retryCount = 0): Promise<void> {
    try {
      await this.handler.handle(event);
    } catch (error) {
      if (retryCount < this.maxRetries) {
        // 재시도 (지수 백오프)
        const delay = Math.pow(2, retryCount) * 1000;
        await this.sleep(delay);
        return this.processEvent(event, retryCount + 1);
      }

      // 최대 재시도 초과: DLQ로 이동
      await this.sendToDLQ(event, error);
    }
  }

  private async sendToDLQ(event: DomainEvent, error: Error): Promise<void> {
    await this.kafka.produce('events.dlq', event.aggregateId, {
      originalEvent: event,
      error: {
        message: error.message,
        stack: error.stack,
      },
      failedAt: new Date().toISOString(),
      retryCount: this.maxRetries,
    });

    // 알림 발송
    await this.alertService.notify(
      `Event processing failed after ${this.maxRetries} retries`,
      { eventId: event.eventId, eventType: event.eventType, error: error.message }
    );
  }
}

{
  "type": "record",
  "name": "OrderPlaced",
  "namespace": "com.ecommerce.orders",
  "fields": [
    {"name": "eventId", "type": "string"},
    {"name": "orderId", "type": "string"},
    {"name": "customerId", "type": "string"},
    {"name": "totalAmount", "type": "long"},
    {"name": "currency", "type": "string", "default": "KRW"},
    {"name": "couponCode", "type": ["null", "string"], "default": null}
  ]
}

import { Kafka, Partitioners } from 'kafkajs';

const kafka = new Kafka({
  clientId: 'order-service',
  brokers: ['kafka-1:9092', 'kafka-2:9092', 'kafka-3:9092'],
  retry: { retries: 5 },
});

const producer = kafka.producer({
  createPartitioner: Partitioners.DefaultPartitioner,
  idempotent: true,
  maxInFlightRequests: 5,
  transactionalId: 'order-producer-1',
});

async function publishOrderEvent(event: OrderPlaced): Promise<void> {
  await producer.send({
    topic: 'events.orders',
    messages: [
      {
        key: event.orderId,
        value: JSON.stringify(event),
        headers: {
          'event-type': event.eventType,
          'event-id': event.eventId,
          'correlation-id': event.metadata.correlationId || '',
        },
      },
    ],
  });
}

const consumer = kafka.consumer({
  groupId: 'payment-service-group',
  sessionTimeout: 30000,
  heartbeatInterval: 3000,
});

async function startConsumer(): Promise<void> {
  await consumer.connect();
  await consumer.subscribe({
    topics: ['events.orders'],
    fromBeginning: false,
  });

  await consumer.run({
    eachMessage: async ({ topic, partition, message }) => {
      const event = JSON.parse(message.value!.toString());
      const eventType = message.headers?.['event-type']?.toString();

      console.log(`Received: ${eventType} from ${topic}[${partition}]`);

      try {
        switch (eventType) {
          case 'order.placed':
            await paymentHandler.onOrderPlaced(event);
            break;
          case 'order.cancelled':
            await paymentHandler.onOrderCancelled(event);
            break;
          default:
            console.log(`Unknown event type: ${eventType}`);
        }
      } catch (error) {
        console.error(`Failed to process event: ${error}`);
        // DLQ 처리 또는 재시도
      }
    },
  });
}

describe('Order Aggregate', () => {
  it('should emit OrderPlaced event when creating order', () => {
    const order = Order.create('ord-1', 'cust-1', [
      { productId: 'prod-1', quantity: 2, unitPrice: 10000 },
    ]);

    const events = order.getUncommittedEvents();
    expect(events).toHaveLength(1);
    expect(events[0]).toBeInstanceOf(OrderPlaced);
    expect((events[0] as OrderPlaced).totalAmount).toBe(20000);
  });

  it('should not allow cancellation of shipped order', () => {
    const order = buildOrderInStatus(OrderStatus.SHIPPED);

    expect(() => order.cancel('Changed mind'))
      .toThrow('Cannot cancel shipped order');
  });
});

describe('Order Saga Integration', () => {
  let kafkaContainer: StartedTestContainer;
  let postgresContainer: StartedTestContainer;

  beforeAll(async () => {
    kafkaContainer = await new GenericContainer('confluentinc/cp-kafka:7.5.0')
      .withExposedPorts(9092)
      .start();

    postgresContainer = await new GenericContainer('postgres:16')
      .withExposedPorts(5432)
      .start();
  });

  it('should complete order saga successfully', async () => {
    // 1. 주문 생성
    const orderId = await orderService.placeOrder({
      customerId: 'cust-1',
      items: [{ productId: 'prod-1', quantity: 1, unitPrice: 10000 }],
    });

    // 2. 이벤트 전파 대기
    await waitForEvent('payment.completed', orderId, 5000);
    await waitForEvent('inventory.reserved', orderId, 5000);

    // 3. 결과 확인
    const order = await orderQueryService.getOrderDetails(orderId);
    expect(order.status).toBe('confirmed');
  });
});